Cannot reproduce in 1.16.5, and I don't believe the bug tracker supports issues in modified servers (Bukkit, Spigot, etc)
Did you have any armor equipped with mending, or an item in your offhand with mending?
Are you certain that no datapack has changed the {{doMobLoot}} gamerule or replaced the loot table of mobs?
This is most likely invalid. What you may be looking for is minecraft.mined
. minecraft.broken
is for the destruction of items via loss of durability (impossible for stained glass).
Technically the hand is not invisible, it's just lowered to a point where you can't see it without swinging your camera very fast.
Edit: I think the cause of the issue is that when attack speed is modified, it seems to jump to the charge that that attack speed would have by that point.
For example, if you set the base to 0 while half-charged, then set the base back to 4, you will likely have a full charge.